Antennas are recognized as being transducers which transduce electromagnetic signals between the guided-wave form, in which the direction of propagation is controlled by a conductive or dielectric waveguide, and a free-space form, in which the propagation takes place in an unguided manner. Those skilled in the art know that many terms associated with antennas are used for historical reasons. For example, at a time at which reception of signals was accomplished only by the use of a long wire connected to the receiver, antenna coupling problems were experienced only when high power was involved, which was the case with transmitting antennas. These antenna coupling problems were discussed in terms of radiation of signals applied to a “feed” point or terminals of the antenna. Only later was it recognized that the radiation pattern and impedance characteristics of antennas were identical regardless of the direction of transduction, but by that time the “feed” terminology was firmly established. Thus, both transmitting and receiving antennas have “feed” points or terminals, and characteristics which are the same. Thus, descriptions of antenna operation may be couched in terms of transmission or reception modes, whichever provides the greater clarity in a given context, with the operation in the other mode being understood from the one description.
Antennas are widely used, to the extent that modern communication and sensing would be unrecognizable without their application. Many antenna types are known, including the long-wire Beverage antenna, the dipole and its monopole-over-ground-plane equivalent. The monopole and dipole antenna linear antennas have well-known radiation and impedance characteristics. Among the radiation pattern characteristics of monopole and dipole antennas are relatively limited bandwidth and relatively low gain, which tend to reduce their usefulness for demanding applications.
The art of arraying of elemental antennas such as monopoles and dipoles has long been used to ameliorate some of the disadvantages of linear antennas. Broadband arrays of dipoles and monopoles are known in the form of one-dimensional or line arrays, which tend to provide greater directivity than a single antenna element. Among the line arrays are log-periodic arrays, in which the dimensions of the constituent antenna elements vary in a monotonic manner along the length of the antenna. Log-periodic arrays, in addition to providing more directive gain than a single linear antenna, also have theoretically unlimited bandwidth. U.S. Pat. Nos. 5,196,857 issued Mar. 23, 1993 in the name of Chiappetta and 5,214,439 issued May 25, 1993 in the name of Reed describe log-periodic arrays. Another widely used type of array is the planar array, which is a two-dimensional arraying of elemental antennas. As ordinarily configured, such planar arrays can provide relatively high directivity in a direction orthogonal to the plane of the array. Those skilled in the art know that many different types of elemental antennas can be arrayed in two dimensions. For example, U.S. Pat. No. 5,258,771 issued Nov. 2, 1993 in the name of Praba describes a two-dimensional array of elemental helix antennas.
For some uses, two-dimensional arrays of antenna elements may include hundreds or even thousands of elemental antennas. The use of elemental antennas in an array may require a transmit-receive (TR) “feed” module for each elemental antenna. One arrangement for implementing transmit-receive feed modules for a two-dimensional array of elemental antennas is described in U.S. Pat. No. 5,017,927, issued May 21, 1991 in the name of Agrawal et al. Another feed arrangement for a two-dimensional antenna array is described in U.S. Pat. No. 5,115,244, issued May 19, 1992 in the name of Freedman et al. Regardless of the type of feed, cost considerations become important when considering arrays of more than a few elements. The cost of the elemental antennas and their ancillary components is of more than passing interest when large numbers of elements are to be used. In this regard, the cost of the individual transmit-receive (TR) modules may be greater than that of the antenna element with which it is associated. Additional cost considerations associated with the use of TR modules relate to the amount of power which the array of modules consumes, which is inextricably linked to the power efficiency of each module. Small increments or decrements in the power consumption of each module can substantially affect the amount of heat-dissipating capability which must be provided for the array, which in turn impacts on the type of mechanical structure required for support and heat conduction. In some cases, liquid coolant paths may be necessary.
While the costs of the TR modules and antenna elements of an array are very important, reliability and performance must be taken into account. When thousands or tens of thousands of elements are used for transduction for generating one or more antenna beams, the TR modules and their connections to control elements must be very reliable. Not only must they be reliable, but the radio-frequency transmissions from each module and element of the array must be in time synchronism.
Improved or alternative antenna array arrangements are desired.